Although it is most common in the over-60s, the brain disorder can strike at any time. Sufferers include actor Michael J Fox, who was just 30 when diagnosed, and former world heavyweight boxing champion Muhammad Ali.

For the latest study, researchers questioned 3,000 men and women, a third of whom had Parkinson's or similar conditions.

The Scottish, Swedish, Italian, Romanian and Maltese volunteers were asked about their use of pesticides such as fly sprays and weedkillers over their lives, both at work and at home.

They were also asked if they had ever been knocked unconscious, if they smoked, and if any family members had Parkinson's.

Analysis of the results revealed that those who had used pesticides the most were 41 per cent more likely to have developed Parkinson's than those who had never been exposed to the chemicals.

Even low levels of exposure raised the risk by 13 per cent.

Those who had suffered repeated blows to the head were more than two and a half times more likely to have developed Parkinson's.

Even a single knock-out blow raised the risk by 35 per cent, the journal Occupational and Environmental Medicine-reported.

Having a family history of Parkinson's also raised the odds of developing the disease.

However, smoking appeared to have a protective effect, a finding which echoes many previous studies.

The reason for this is thought to be that the balance of chemicals in the brain that leads to people becoming addicted to cigarettes also makes them less likely to develop Parkinson's - not that any substance in cigarettes wards off disease.

Dr Kieran Breen, of the Parkinson's Disease Society, said: "The important finding from this study is that Parkinson's is not caused by any one factor, but instead a combination of genetic susceptibility and environmental factors."

Peter Sanguinetti, of the Crop Protection Association, which represents pesticide manufacturers, said that product safety is a priority.

He stressed, however, that the latest study did not find conclusive evidence of a link between pesticides and Parkinson's.